1. Field of the Invention
The invention relates to a process for the preparation of an elastomer-containing vinyl chloride graft copolymer having a high bulk density and very good flow by the suspension polymerization process.
2. Description of the Related Art
Processes for the preparation of elastomercontaining graft copolymers using vinyl chloride as the grafting monomer are known.
A process for the preparation of polyacrylatecontaining graft copolymers of vinyl chloride by suspension polymerization in the presence of a protective colloid and a H.sub.2 O or oil-soluble initiator, the graft base being added as an emulsion, is described in DE-AS 1 082 734. The process according to DE-AS 1 090 856 describes an improved process variant of the above Auslegeschrift. Vinyl and allyl compounds which are copolymerizable with vinyl chloride are also employed as grafting monomers in order to elastify the PVC content of the graft copolymer. Another process variant using a crosslinked graft base is claimed in DE-AS 1 090 857.
However, a disadvantage of all these processes is that polymer deposits form on the wall surfaces and equipment components of the polymerization reactor in the course of the polymerization, especially if the elastomer content in the graft copolymer is relatively high. The product moreover has a wide particle size distribution. The polymer tends to stick, has poor flow properties and has only a low bulk density.
A process in which vinyl acetate/ethylene copolymer in the solid state is employed as the graft base is claimed in DE-A 1 495 694 (U.S. Pat. No. 3,358,054). The solid VAE copolymer is dissolved in the vinyl chloride monomer phase, the solution is dispersed in water and grafting is carried out by the suspension polymerization process. DE-A 1 495 802 is an Application for a Patent of Addition to DE-A 1 495 694, in which, to standardize the particle size distribution, the weight ratio of the aqueous phase to the sum of the graft substrate and grafting monomer has been reduced to 1.5 : 1 to 0.5 : 1. A disadvantage of this process is the time-consuming and complicated step of dissolving the solid resin. Because of the rapidly rising viscosity of the solution, it is difficult to achieve solid resin contents above 15%. Inhomogeneities in respect of the graft copolymer composition, which lead to a considerable reduction in product quality, furthermore occur in this process, especially in the case of short dissolving times for the solid resin.
EP-A 195 942 relates to a process for the preparation of a free-flowing polyacrylate/vinyl chloride graft copolymer by suspension polymerization in the presence of methylcellulose as a protective colloid. The graft base, which is present in emulsion, is initially introduced into the vessel together with the protective colloid, coagulation is carried out by addition of a Ca(OH).sub.2 suspension, and after neutralization, grafting is carried out, the peroxide initiator being added and vinyl chloride being forced in. The disadvantage of this process is that only relatively coarse products of low bulk density can be obtained by it.
U.S. Pat. No. 4,208,490 claims a process for the preparation of graft copolymers in which the elastomeric graft substrate is initially introduced into the vessel as an emulsion, together with the ethylenically unsaturated grafting monomer and the initiator, and the polymerization is conducted as an emulsion polymerization up to a conversion of 5% of the grafting monomer. Only then, after addition of the protective colloid, is the polymerization continued as a suspension polymerization, without the graft base added as an emulsion being coagulated.
The object was therefore to develop a process for the preparation of elastomer-containing vinyl chloride graft copolymers having a high elastomer content of up to 65% by weight without the disadvantages described above in respect to product quality and processability. Above all, the graft copolymer should have a high bulk density, very good flow properties and a homogeneous distribution of the flexible phase.
Surprisingly, this has been achieved by the metering process according to the invention and by the use of a specific protective colloid.